How To Prioritize Tree Plantings

There are a large amount of considerations and priorities when deciding to plant a tree.

It would be great if budgets were unlimited and wherever and whenever trees were needed, they would be available.

Unfortunately, we don’t live in utopia and, as such, must use real-world solutions to the problems we face. One such issue is how to incorporate woody plants into a landscape, commercial property or recreational facility to maximize function and aesthetic value without breaking the bank. There are many pertinent considerations.

Consider the master plan and purpose of the tree when replanting after a storm. Photo: John Fech

Guidelines for prioritization

In the prioritization process, the first step is to identify the tree’s purpose. Is it for fall color? Is it for erosion control on a slope? Is it to replace a screening tree? The function of the tree will help guide all other decisions and considerations going forward. It’s all too common for trees to be replaced automatically without a thought as to why they are there in the first place. If there is no real purpose or reason for the tree to be in a particular location, then, chances are, the money could be better spent somewhere else.

The next concern is the current condition — the tree’s state of health and vigor. Many defects are easy to spot, while others can be hidden. The classic weaknesses of cracks, co-dominant leaders, basal/root plate disorders, heartwood decay and leaning are relatively easy to spot. The other three — insects, diseases and lack of favorable growing conditions — are more difficult to analyze in that further investigation often is necessary to evaluate how they contribute to the tree’s condition.

On top of identifying a defect or malady, one must determine its extent. A small pocket of decay is not a particularly troublesome problem, whereas a tree that is leaning to the west accompanied by a pushed-up mound of soil on the east side is a pretty heavy-duty concern. These considerations help quantify the degree of priority that should be placed on keeping or replacing a specimen and are related to the probability of failure (PoF) as referred to by the International Society of Arboriculture. During evaluation and assessment, each tree is given a PoF value of 1 to 5, or categorized as unlikely, moderately probable or likely to fail based on all the defects, pests and growing conditions.

Common high-priority scenarios

Right from the start: In the situation of new projects, residential properties and sports turf/golf course/park plantings, a lack of trees can be seen as a positive in that a decent amount of time can be spent on planning for location and purpose rather than removal and replacement. This scenario is the proverbial “clean slate.” As such, program statements that define the objectives for eventual contributions to the property should be devised and clearly stated. Examples include “incorporate a mixed planting of deciduous and evergreen trees to provide screening, wildlife habitat and background scale for year-round human activity” and “create a natural setting in which trees are of high, medium and low heights on the edge of recreational spaces for picnics and other recreational activity.” These statements provide the overall context of a master plan, offering lots of foresight to a new planting.

After a storm: Following a major weather event, the appropriate action step is one of balancing the evaluation of condition of individual trees, the intended purpose, desirable tree availability and the funds available for replanting. In fact, a reasonable approach is to move forward in that order — evaluation, recall of purpose, determining availability of tree stock and labor and obtaining funds. The latter part of the protocol is much easier to accomplish if the former are finished before talking to stakeholders and officials responsible for approving expenditures. Having a plan in place and assessment of inventory is just doing your homework.

Obvious defects, such as heartwood decay and co-dominant leaders, make prioritization a little easier. Photo: John Fech

Gradually: During disease and insect outbreaks such as apple scab, bagworms and diplodia tip blight, trees die steadily over a period of a few years. This scenario differs from the previous one in that it is generally uncertain if all the trees of a certain species will be affected. Weather conditions often encourage the development of certain insects and diseases, but the healthiest trees often will resist a pest invasion. As trees die, priority should be focused on replacement, but in the context of changes that might have occurred since the landscape was installed as well as program statements that guide the outcome. In the case of more aggressive maladies, such as emerald ash borer and pine wilt, the scenario changes to become more of a hybrid between gradual and the aftermath of a storm. The timeframe on these situations tends to be one of a few years rather than once in a while. Nonetheless, the considerations of importance on function and amenity in the landscape are still crucial to the success of the process.

Using several species of oak adds valuable diversity to a property. Photo: John Fech

Diversity plantings: Sometimes, when a particular property has been inventoried and a high percentage of a certain species is noted, diversification becomes a priority. Increased diversity on a property usually results in greater resistance to losses from pests and weather-related occurrences. General diversity in the landscape is a laudable outcome. A former guideline was to have no more than 10 to 15 percent of a particular genus represented among tree specimens. An updated recommendation is to further diversify, carrying it down to the species level. For example, an attempt should be made to include several species of a particular genus in a planting, i.e. Shingle oak, chinkapin oak, swamp white oak and red oak instead of utilizing only sawtooth oak. As well, the “value” of a tree can be taken into account. Some species, such as mulberry and Siberian elm, are generally considered to be of lower value. In most cases, these trees were planted from seed by nature rather than intentionally planned, thus underscoring the need for considering the overall diversity of the planting.

Memorials: One category that can be difficult to manage is memorial or historic tree replacement prioritization. In addition to function, condition, amenity, intended purpose and diversity deliberations, this scenario adds the element of distinctive human emotion to the picture. When a tree that is merely one of a dozen that need replacement fails, it is usually considered strictly in terms of its value as a living organism.

When the factor of the trees’ origin and/or age is added, prioritization becomes more complicated. Stakeholders tend to be less supportive of replacement when the 100-year-old walnut is slated for removal or a tree planted in honor of a major donor is struck by lightning. To the extent possible, keep the memorial aspect of a tree in perspective. Yes, it is an important factor, but ultimately, the physics, health and vigor of a specimen must be worthy enough to support its retention on a property. In such cases, pointing out that lawsuits and insurance claims can occur if due diligence is not taken may help in tree prioritization.

Editor’s note: This article was originally published May 2015 and has been updated.